12 Essential SOLID Principle Interview Questions Every Developer Should Know

Been to a few technical interviews lately? You’ve probably faced questions about SOLID principles. These five design principles - Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion, are the foundation of clean object-oriented design.

Interviewers ask about SOLID because it shows how you think about code structure and trade-offs. They’re not just textbook concepts, they’re actual tools that separate people who write working code from those who write maintainable code.

Let’s go through 12 common SOLID interview questions. I’ll share straightforward answers, real C# examples, and the Follow-Up Questions You Might Be Asked interviewers often ask. Think of this as practical interview prep that actually helps you build better systems.

1. What are the SOLID principles, and why are they important in Object-Oriented Design?

Answer: SOLID is an acronym for five design principles that help create maintainable, testable, and flexible object-oriented code:

• Single Responsibility Principle (SRP)
• Open/Closed Principle (OCP)
• Liskov Substitution Principle (LSP)
• Interface Segregation Principle (ISP)
• Dependency Inversion Principle (DIP)

These principles matter because they address the core problems in software development: code that’s hard to change, test, and understand. When you follow SOLID, you’re building systems that can evolve without breaking existing functionality.

graph LR

    OOP[Object-Oriented Programming] --> Encapsulation
    OOP --> Inheritance
    OOP --> Polymorphism
    OOP --> Abstraction

    subgraph SOLID[**SOLID Principles**]
        SRP[Single Responsibility Principle]
        OCP[Open/Closed Principle]
        LSP[Liskov Substitution Principle]
        ISP[Interface Segregation Principle]
        DIP[Dependency Inversion Principle]
    end

    Encapsulation --> SRP
    Inheritance --> LSP
    Inheritance --> OCP
    Polymorphism --> OCP
    Abstraction --> ISP
    Abstraction --> DIP

    

Follow-Up Questions You Might Be Asked:

• “Can you give an example of technical debt caused by violating these principles?” Tip: Mention a specific example like a God class that handles UI, business logic, and data access, making changes risky and testing nearly impossible.

• “How do SOLID principles relate to other design patterns you’ve used?” Tip: Explain how patterns like Strategy, Factory, and Observer naturally implement SOLID principles in practice.

Common Things People Get Wrong:

• Treating SOLID as rigid rules rather than guidelines
• Over-engineering simple solutions to follow SOLID
• Not understanding the trade-offs between principles

How This Shows Up in Real Life: Use SOLID as a code review checklist. When you spot a class doing multiple things, ask if it breaks SRP. When touching existing code for new features, think about whether OCP could save you future headaches.

Why It Matters:
SOLID is not theory, it’s a sanity checklist. These 5 principles guide how to write code that evolves without breaking everything else.

2. Explain the Single Responsibility Principle (SRP) with a real-world code example

Answer: SRP states that a class should have only one reason to change. In practical terms, each class should focus on a single concern or responsibility.

Here’s a common violation:

// BAD: Multiple responsibilities in one class
public class UserService
{
    public void RegisterUser(User user)
    {
        // Validation
        if (string.IsNullOrEmpty(user.Email))
            throw new ArgumentException("Email is required");

        // Email sending
        var smtp = new SmtpClient();
        smtp.Send(user.Email, "Welcome!", "Thanks for signing up!");
    }
}

Better approach following SRP:

// GOOD: SRP-compliant
public class UserService
{
    private readonly IUserValidator _validator;
    private readonly IEmailService _emailService;

    public UserService(IUserValidator validator, IEmailService emailService)
    {
        _validator = validator;
        _emailService = emailService;
    }

    public void RegisterUser(User user)
    {
        _validator.Validate(user);
        _emailService.SendWelcome(user.Email);
    }
}
public class UserValidator : IUserValidator
{
    public void Validate(User user)
    {
        if (string.IsNullOrEmpty(user.Email))
            throw new ArgumentException("Email is required");
    }
}

Follow-Up Questions You Might Be Asked:

• “How do you identify when a class violates SRP?” Tip: Look for words like “and” in class descriptions. If it’s doing multiple things, it’s likely violating SRP. Also watch for methods that modify unrelated fields.

• “What are the trade-offs of having many small classes vs. fewer larger ones?” Tip: Small classes are focused but can lead to “class explosion” and navigation challenges. Larger classes reduce overhead but risk becoming unmaintainable.

Common Things People Get Wrong:

• Creating classes that are too granular (one method per class)
• Confusing SRP with having only one method
• Not considering cohesion when splitting responsibilities

One Class, One Job:
If a class handles logic and data access and emails, you’re heading into God class territory. SRP keeps responsibilities focused and testable.

Want a deeper dive into SRP? Read the full SRP post here.

3. What’s the difference between the Open/Closed Principle (OCP) and the Strategy Pattern? How does OCP support extensibility?

Answer: OCP states that software entities should be open for extension but closed for modification. The Strategy Pattern is one way to implement OCP, but they’re not the same thing.

OCP is the principle; Strategy is a design pattern that helps achieve it. OCP can be implemented through inheritance, composition, interfaces, or dependency injection.

flowchart TB
  subgraph subGraph0["OCP Applied"]
    Client["Client Code"]
    PaymentService["PaymentService"]
    IPaymentProcessor["IPaymentProcessor"]
  end

  subgraph subGraph1["Strategies"]
    CreditCard["CreditCardProcessor"]
    PayPal["PayPalProcessor"]
  end

  Client -->|Injects| PaymentService
  PaymentService -->|Depends on| IPaymentProcessor
  IPaymentProcessor --> CreditCard
  IPaymentProcessor --> PayPal


    

Here’s how OCP looks in a real system where different payment methods are needed:

public interface IPaymentProcessor
{
    Task<PaymentResult> ProcessAsync(decimal amount, PaymentDetails details);
}

public class PaymentService
{
    private readonly IPaymentProcessor _processor;
    public PaymentService(IPaymentProcessor processor) => _processor = processor;

    public Task<PaymentResult> ProcessPaymentAsync(decimal amount, PaymentDetails details) =>
        _processor.ProcessAsync(amount, details);
}

public class CreditCardProcessor : IPaymentProcessor
{
    public Task<PaymentResult> ProcessAsync(decimal amount, PaymentDetails details) =>
        Task.FromResult(new PaymentResult { Success = true });
}

public class PayPalProcessor : IPaymentProcessor
{
    public Task<PaymentResult> ProcessAsync(decimal amount, PaymentDetails details) =>
        Task.FromResult(new PaymentResult { Success = true });
}

Follow-Up Questions You Might Be Asked:

• “How do you balance OCP with YAGNI (You Aren’t Gonna Need It)?” Tip: Apply OCP only when you know changes are coming. For areas with stable requirements, stick with simpler code until you actually need flexibility.

• “Can you show an example where OCP led to over-engineering?” Tip: Describe a validation system with complex abstractions that ultimately only had one implementation. Sometimes direct code is better than unnecessary indirection.

Common Things People Get Wrong:

• Creating abstractions for everything “just in case”
• Confusing OCP with never changing existing code
• Not considering the cost of abstraction vs. future flexibility

How This Shows Up in Real Life: Use OCP when you know features will keep growing. Payment methods, notification channels, and export formats are perfect examples. Skip it for stable code that rarely changes.

OCP = What, Strategy = How:
OCP says don’t touch existing code to add new behavior. Strategy Pattern is one way to make that happen using polymorphism.

Read the full OCP breakdown here.

4. How do you apply the Liskov Substitution Principle (LSP)? Can you give an example of an LSP violation?

Answer: LSP states that objects of a superclass should be replaceable with objects of a subclass without altering the correctness of the program. In simpler terms: if you have a base class or interface, any implementation should work the same way from the caller’s perspective.

LSP violation example:

public abstract class Bird
{
    public abstract void Fly();
}

public class Sparrow : Bird
{
    public override void Fly() => Console.WriteLine("Sparrow flying");
}

public class Penguin : Bird
{
    public override void Fly() => throw new NotSupportedException("Penguins can't fly");
}

public void MakeBirdFly(Bird bird)
{
    bird.Fly(); // Will crash for Penguin
}

Better approach following LSP:

public abstract class Bird
{
    public abstract void Move();
}

public interface IFlyable
{
    void Fly();
}

public class Sparrow : Bird, IFlyable
{
    public override void Move() => Fly();
    public void Fly() => Console.WriteLine("Sparrow flying");
}

public class Penguin : Bird
{
    public override void Move() => Swim();
    public void Swim() => Console.WriteLine("Penguin swimming");
}
public void MakeBirdMove(Bird bird)
{
    bird.Move(); // Works for both Sparrow and Penguin
}

Now each class has consistent behavior, and MakeBirdMove(Bird bird) won’t crash regardless of the bird type.

Follow-Up Questions You Might Be Asked:

• “How does LSP relate to the concept of ‘is-a’ relationships?” Tip: Not all “is-a” relationships should be inheritance. A Square “is-a” Rectangle mathematically, but behaviorally they’re different in OOP. Use composition when behavioral substitution fails.

• “Can you give a real-world example from your experience?” Tip: Share a specific case where you refactored inheritance to use composition or interfaces to fix unexpected behavior during substitution.

Common Things People Get Wrong:

• Using inheritance when composition is more appropriate
• Creating contracts that are too restrictive or too permissive
• Not considering preconditions and postconditions

Replace Without Fear:
LSP means subclasses shouldn’t break expectations. If swapping a base class with a child breaks things , rethink your hierarchy.

Want a deeper dive? Full LSP breakdown

5. Describe Interface Segregation Principle (ISP). How is it different from SRP?

Answer: ISP states that no client should be forced to depend on interfaces they don’t use. It’s about creating focused, client-specific interfaces rather than large, monolithic ones.

While SRP focuses on classes having one responsibility, ISP focuses on interfaces being client-specific. A class might have multiple interfaces if it serves different clients.

flowchart TD
    subgraph BadDesign["Violates ISP"]
        FatInterface["IUserManager" <br/> Get, Save, Delete, Export, Email, Validate]
        ClientA["UserProfileService"] --> FatInterface
        ClientB["AdminPanelService"] --> FatInterface
    end

    subgraph GoodDesign["Follows ISP"]
        Reader["IUserReader"]
        Writer["IUserWriter"]
        Notifier["IUserNotifier"]
        ClientA2["UserProfileService"] --> Reader
        ClientB2["AdminPanelService"] --> Writer
        ClientB2 --> Notifier
    end

    

// BAD: Fat interface forces unnecessary dependencies
public interface IUserManager
{
    Task<User> GetUserAsync(int id);
    Task SaveUserAsync(User user);
    Task DeleteUserAsync(int id);
    Task<byte[]> ExportUserReportAsync();
    Task SendWelcomeEmailAsync(int userId);
    Task ValidateUserPermissionsAsync(int userId, string action);
}

public class UserProfileService
{
    private readonly IUserManager _userManager;
    
    public UserProfileService(IUserManager userManager)
    {
        _userManager = userManager; // Depends on methods it doesn't need
    }

    public async Task<User> GetProfileAsync(int userId)
    {
        return await _userManager.GetUserAsync(userId);
        // Doesn't need export, email, or permission methods
    }
}

Better approach following ISP:

// GOOD: Segregated interfaces
public interface IUserReader
{
    Task<User> GetUserAsync(int id);
}

public interface IUserWriter
{
    Task SaveUserAsync(User user);
    Task DeleteUserAsync(int id);
}

public interface IUserReporter
{
    Task<byte[]> ExportUserReportAsync();
}

public interface IUserNotifier
{
    Task SendWelcomeEmailAsync(int userId);
}

// Implementation can implement multiple interfaces
public class UserManager : IUserReader, IUserWriter, IUserReporter, IUserNotifier
{
    public async Task<User> GetUserAsync(int id) { /* implementation */ }
    public async Task SaveUserAsync(User user) { /* implementation */ }
    public async Task DeleteUserAsync(int id) { /* implementation */ }
    public async Task<byte[]> ExportUserReportAsync() { /* implementation */ }
    public async Task SendWelcomeEmailAsync(int userId) { /* implementation */ }
}

// Clients only depend on what they need
public class UserProfileService
{
    private readonly IUserReader _userReader;
    
    public UserProfileService(IUserReader userReader)
    {
        _userReader = userReader; // Clean, focused dependency
    }

    public async Task<User> GetProfileAsync(int userId)
    {
        return await _userReader.GetUserAsync(userId);
    }
}

Follow-Up Questions You Might Be Asked:

• “How do you balance ISP with having too many small interfaces?” Tip: Group related operations that clients typically use together. Watch actual client usage patterns. Role interfaces often make more sense than method-per-interface approaches.

• “When might you violate ISP intentionally?” Tip: When working with third-party APIs or legacy systems where you can’t change the interfaces. Sometimes a single “fat” interface with adapter implementations is more practical than perfect ISP.

Common Things People Get Wrong:

• Creating interfaces that are too granular (one method per interface)
• Not considering the actual clients when designing interfaces
• Confusing ISP with SRP (they work together but are different)

Quick Difference: SRP vs ISP

• SRP (Single Responsibility Principle) is about classes having only one reason to change - one focused responsibility.
• ISP (Interface Segregation Principle) is about interfaces being client-specific - no class should be forced to implement methods it doesn’t use.

Want a deeper dive? Read the full ISP breakdown here.

6. Explain Dependency Inversion Principle (DIP). How is it different from Dependency Injection?

Answer: DIP states that high-level modules should not depend on low-level modules. Both should depend on abstractions. Also, abstractions should not depend on details; details should depend on abstractions.

Dependency Injection is a technique to implement DIP, but they’re not the same thing. DIP is about the direction of dependencies; DI is about how you provide those dependencies.

flowchart TD
    subgraph Without_DIP["Without DIP"]
        A1((High-Level Module<br/>OrderService)) -->|Direct Dependency| B1((Low-Level Module<br/>SqlRepository))
    end

    subgraph With_DIP["With DIP + DI"]
        A2((High-Level Module<br/>OrderService)) -->|Depends on| C((Interface<br/>IOrderRepository))
        B2((Low-Level Module<br/>SqlRepository)) -->|Implements| C
        D((DI Container<br/>or Constructor)) --> A2
        D --> B2
    end

    

DIP violation:

// BAD: High-level module depends on concrete classes
public class OrderService
{
    private readonly SqlServerRepository _repository = new();
    private readonly SmtpEmailService _emailService = new();

    public async Task ProcessOrderAsync(Order order)
    {
        await _repository.SaveAsync(order);
        await _emailService.SendConfirmationAsync(order.CustomerEmail);
    }
}

Following DIP with Dependency Injection:

// GOOD: Depends on abstractions, uses DI for implementation
public class OrderService
{
    private readonly IOrderRepository _repository;
    private readonly IEmailService _emailService;

    public OrderService(IOrderRepository repository, IEmailService emailService)
    {
        _repository = repository;
        _emailService = emailService;
    }

    public async Task ProcessOrderAsync(Order order)
    {
        await _repository.SaveAsync(order);
        await _emailService.SendConfirmationAsync(order.CustomerEmail);
    }
}

// Low-level modules depend on abstractions
public class SqlServerRepository : IOrderRepository
{
    public async Task SaveAsync(Order order)
    {
        // SQL Server specific implementation
    }
}

public class SendGridEmailService : IEmailService
{
    public async Task SendConfirmationAsync(string email)
    {
        // SendGrid specific implementation
    }
}

// DI Container configuration
services.AddScoped<IOrderRepository, SqlServerRepository>();
services.AddScoped<IEmailService, SendGridEmailService>();
services.AddScoped<OrderService>();

Follow-Up Questions You Might Be Asked:

• “What are the benefits of inverting dependencies?” Tip: It protects business logic from infrastructure changes and makes upgrades (like switching databases) much easier.

• “How does DIP affect testing?” Tip: It makes unit testing possible by letting you swap real dependencies with test doubles.

• “Can you show an example without using a DI container?” Tip: Simple constructor injection works without containers. Factories or service locator patterns are other options, though they have their own trade-offs.

Common Things People Get Wrong:

• Thinking DI containers are required for DIP
• Creating abstractions that are too specific to one implementation
• Not considering the ownership of interfaces (they should belong to the client)

How This Shows Up in Real Life: Use DIP for anything you want to test properly or for things that might change later. It’s especially helpful with outside dependencies like databases, APIs, and file systems.

DIP vs DI:
DIP is the principle - “depend on abstractions, not concrete types.”
DI is the technique - constructor, setter, or framework injection.
You can follow DIP without using a DI container.

Want a deeper dive? Read the full post on Dependency Inversion Principle here.

Confused about DI vs DIP vs IoC?
This post explains the difference clearly →

7. How do SOLID principles improve maintainability and testability in large-scale applications?

Answer: SOLID principles address the main challenges in large applications: complexity, tight coupling, and difficulty in making changes. Here’s how each principle helps:

Maintainability Benefits:

• SRP: Changes to one feature don’t affect unrelated functionality
• OCP: New features can be added without modifying existing code
• LSP: Polymorphism works correctly, making code predictable
• ISP: Changes to interfaces affect fewer clients
• DIP: Business logic is isolated from implementation details

Testability Benefits:

// Example: Testing becomes easier with SOLID
public class OrderProcessingService
{
    private readonly IOrderRepository _orderRepository;
    private readonly IPaymentService _paymentService;
    private readonly IInventoryService _inventoryService;
    private readonly INotificationService _notificationService;

    public OrderProcessingService(
        IOrderRepository orderRepository,
        IPaymentService paymentService,
        IInventoryService inventoryService,
        INotificationService notificationService)
    {
        _orderRepository = orderRepository;
        _paymentService = paymentService;
        _inventoryService = inventoryService;
        _notificationService = notificationService;
    }

    public async Task<OrderResult> ProcessOrderAsync(Order order)
    {
        var inventoryResult = await _inventoryService.ReserveItemsAsync(order.Items);
        if (!inventoryResult.Success)
            return OrderResult.Failed("Insufficient inventory");

        var paymentResult = await _paymentService.ProcessPaymentAsync(order.Payment);
        if (!paymentResult.Success)
        {
            await _inventoryService.ReleaseItemsAsync(order.Items);
            return OrderResult.Failed("Payment failed");
        }

        await _orderRepository.SaveAsync(order);
        await _notificationService.SendOrderConfirmationAsync(order);
        
        return OrderResult.Success();
    }
}

// Easy to test with mocks
[Test]
public async Task ProcessOrder_WhenInventoryInsufficient_ShouldReturnFailure()
{
    // Arrange
    var mockInventory = new Mock<IInventoryService>();
    mockInventory.Setup(x => x.ReserveItemsAsync(It.IsAny<List<OrderItem>>()))
               .ReturnsAsync(InventoryResult.Failed("Out of stock"));

    var service = new OrderProcessingService(
        Mock.Of<IOrderRepository>(),
        Mock.Of<IPaymentService>(),
        mockInventory.Object,
        Mock.Of<INotificationService>());

    // Act
    var result = await service.ProcessOrderAsync(new Order());

    // Assert
    Assert.False(result.Success);
    Assert.Contains("Out of stock", result.ErrorMessage);
}

Follow-Up Questions You Might Be Asked:

• “Can you give an example of how SOLID helped in a specific project?” Tip: Share a concrete story about how applying a specific SOLID principle (like DIP) made adding a new feature much easier than expected.

• “What metrics do you use to measure maintainability?” Tip: Mention practical metrics like time to implement changes, defect rates after changes, and code coverage. Also discuss qualitative signs like developer confidence when changing code.

Common Things People Get Wrong:

• Over-engineering simple applications
• Not considering the team’s experience level
• Applying all principles everywhere regardless of context

TL;DR: SOLID keeps code parts properly separated and focused. This means faster feature development, tests that actually work, and fewer weird bugs in big projects.

Scale Without Chaos:
SOLID keeps complexity in check. It ensures that as systems grow, they don’t collapse under their own weight.

8. What is a practical example where breaking a SOLID principle was justified?

Answer: Sometimes breaking SOLID principles is the right choice. Here are practical scenarios:

Performance-Critical Code:

// SRP Violation: All logic bundled for ultra-low latency
public class HighFrequencyTradingEngine
{
    private readonly MarketDataBuffer _buffer = new(10000);
    private readonly OrderBook _orderBook = new();
    private readonly RiskCalculator _riskCalculator = new();

    public void ProcessMarketData(MarketData data)
    {
        _buffer.Add(data);
        var orders = _orderBook.GetMatchingOrders(data);
        var risk = _riskCalculator.CalculateRisk(orders);
        if (risk.IsAcceptable)
            ExecuteOrders(orders);
    }
}

Keeping everything local avoids performance hits from abstraction or indirection.

Simple CRUD Operations:

// DIP Violation: Directly using DbContext for simple logic
public class UserController : ControllerBase
{
    private readonly ApplicationDbContext _context;

    public UserController(ApplicationDbContext context)
    {
        _context = context;
    }

    [HttpGet("{id}")]
    public async Task<User> GetUser(int id)
    {
        return await _context.Users.FindAsync(id);
    }
}

Adding abstraction here would be unnecessary overhead.

Legacy Integration:

// Breaking LSP when working with legacy systems
public abstract class LegacyReportGenerator
{
    public abstract string GenerateReport();
}

public class ModernReportGenerator : LegacyReportGenerator
{
    public override string GenerateReport()
    {
        // Modern implementation
        return JsonSerializer.Serialize(GetReportData());
    }
}

public class LegacyXmlReportGenerator : LegacyReportGenerator
{
    public override string GenerateReport()
    {
        // Legacy system expects XML, not JSON
        // This violates LSP but is necessary for compatibility
        return GenerateXmlReport();
    }
}

Sometimes you break substitution to meet unavoidable external expectations.

Follow-Up Questions You Might Be Asked:

• “How do you decide when to break SOLID principles?” Tip: Break them when you have concrete evidence that following them would cause serious issues (like performance problems). Always measure before optimizing.

• “What safeguards do you put in place when breaking these principles?” Tip: Isolate violations behind clean interfaces, document why you made the decision, and add tests around the violation to prevent regressions.

Common Things People Get Wrong:

• Using performance as an excuse without measuring
• Breaking principles out of laziness rather than necessity
• Not documenting why the principle was broken

How This Shows Up in Real Life: Add comments when you break SOLID principles on purpose. Future developers (including future you) will thank you for explaining why you made the call instead of just assuming it was a mistake.

Break With Purpose:
SOLID isn’t religion - it’s a guide. If you must break it, make the tradeoff explicit, isolate the risk, and document the decision clearly.

9. How do SOLID principles relate to Clean Architecture or Hexagonal Architecture?

Answer: SOLID principles are the foundation that makes Clean Architecture and Hexagonal Architecture work. These architectural patterns are essentially large-scale applications of SOLID principles.

flowchart TD
 subgraph subGraph0["Presentation Layer"]
        Controller["UI / API Controller"]
  end
 subgraph subGraph1["Application Layer"]
        AppService["OrderService<br>(Use Case)"]
  end
 subgraph subGraph2["Domain Layer"]
        DomainEntity["Order<br>(Entity)"]
        DomainInterface["IOrderRepository<br>(Interface)"]
  end
 subgraph subGraph3["Infrastructure Layer"]
        SqlRepo["SqlOrderRepository"]
        EmailService["SendGridEmailService"]
  end
    Controller --> AppService
    AppService --> DomainInterface & DomainEntity
    SqlRepo -- implements --> DomainInterface
    EmailService -- used by --> AppService

    

Clean Architecture Structure:

// Domain Layer (Core) - No dependencies on external layers
public class Order
{
    public int Id { get; set; }
    public List<OrderItem> Items { get; set; } = new();
    public decimal Total => Items.Sum(x => x.Price * x.Quantity);
    
    public void AddItem(OrderItem item)
    {
        Items.Add(item);
    }
}

public interface IOrderRepository // Domain owns the interface (DIP)
{
    Task<Order> GetByIdAsync(int id);
    Task SaveAsync(Order order);
}

// Application Layer - Uses domain interfaces
public class OrderService
{
    private readonly IOrderRepository _orderRepository;
    private readonly IPaymentService _paymentService;

    public OrderService(IOrderRepository orderRepository, IPaymentService paymentService)
    {
        _orderRepository = orderRepository;
        _paymentService = paymentService;
    }

    public async Task<OrderResult> ProcessOrderAsync(CreateOrderRequest request)
    {
        var order = new Order();
        // Business logic here
        
        var paymentResult = await _paymentService.ProcessPaymentAsync(order.Total);
        if (paymentResult.Success)
        {
            await _orderRepository.SaveAsync(order);
        }
        
        return new OrderResult { Success = paymentResult.Success };
    }
}

// Infrastructure Layer - Implements domain interfaces
public class SqlOrderRepository : IOrderRepository
{
    private readonly ApplicationDbContext _context;

    public SqlOrderRepository(ApplicationDbContext context)
    {
        _context = context;
    }

    public async Task<Order> GetByIdAsync(int id)
    {
        return await _context.Orders.FindAsync(id);
    }

    public async Task SaveAsync(Order order)
    {
        _context.Orders.Add(order);
        await _context.SaveChangesAsync();
    }
}

// Presentation Layer - Depends on application layer
[ApiController]
[Route("api/[controller]")]
public class OrdersController : ControllerBase
{
    private readonly OrderService _orderService;

    public OrdersController(OrderService orderService)
    {
        _orderService = orderService;
    }

    [HttpPost]
    public async Task<IActionResult> CreateOrder(CreateOrderRequest request)
    {
        var result = await _orderService.ProcessOrderAsync(request);
        return result.Success ? Ok(result) : BadRequest(result);
    }
}

How SOLID Enables Clean Architecture:

• SRP: Each layer has a single responsibility
• OCP: New features extend without modifying core layers
• LSP: Implementations can be swapped without breaking contracts
• ISP: Interfaces are client-specific (domain defines what it needs)
• DIP: Dependencies flow inward (infrastructure depends on domain)

Follow-Up Questions You Might Be Asked:

• “How do you handle cross-cutting concerns in Clean Architecture?” Tip: Use decorators, middlewares, or aspects that wrap core services without violating the dependency rule. For logging or caching, create abstraction in the domain layer implemented in outer layers.

• “What are the trade-offs of using Clean Architecture?” Tip: The initial cost is higher and there’s more code to write. For simple CRUD apps, it might be overkill. But for complex business logic or long-term projects, the maintainability benefits outweigh the setup costs.

Common Things People Get Wrong:

• Creating too many layers for simple applications
• Not understanding dependency directions
• Putting too much logic in the application layer

TL;DR: Clean Architecture is just SOLID applied to your entire system. Each layer has a single job, and dependencies point inward. It’s not a new invention - it’s structured, large-scale object-oriented thinking.

Clean Architecture = SOLID at Scale
SRP, DIP, and ISP aren’t just for classes - they’re how your whole architecture stays testable, flexible, and decoupled.

10. What common mistakes do developers make when trying to apply SOLID?

Answer: Here are the most common SOLID mistakes I’ve seen in code reviews:

1. Over-Engineering Simple Solutions:

// BAD: Unnecessary abstraction for simple logic
public interface IAdditionCalculator
{
    int Add(int a, int b);
}

public class BasicAdditionCalculator : IAdditionCalculator
{
    public int Add(int a, int b)
    {
        return a + b;
    }
}

// GOOD: Keep it simple
public static class Calculator
{
    public static int Add(int a, int b) => a + b;
}

2. Creating Anemic Interfaces:

// BAD: Interface too specific to one implementation
public interface ISqlUserRepository
{
    Task<User> GetUserByIdAsync(int id);
    Task<List<User>> GetUsersByConnectionStringAsync(string connectionString);
    Task ExecuteSqlQueryAsync(string sql);
}

// GOOD: Technology-agnostic interface
public interface IUserRepository
{
    Task<User> GetByIdAsync(int id);
    Task<List<User>> GetAllAsync();
    Task<User> GetByEmailAsync(string email);
}

3. Misunderstanding LSP:

// BAD: Violating LSP by changing behavior
public class Rectangle
{
    public virtual int Width { get; set; }
    public virtual int Height { get; set; }
}

public class Square : Rectangle
{
    public override int Width 
    { 
        get => base.Width; 
        set => base.Width = base.Height = value; 
    }
    public override int Height 
    { 
        get => base.Height; 
        set => base.Width = base.Height = value; 
    }
}

// This breaks LSP - client expectations are violated
public void TestRectangle(Rectangle rect)
{
    rect.Width = 5;
    rect.Height = 10;
    Assert.AreEqual(50, rect.Width * rect.Height); // Fails for Square
}

4. Interface Explosion (ISP Gone Wrong):

// BAD: Too many tiny interfaces
public interface IUserCreator
{
    Task CreateUserAsync(User user);
}

public interface IUserUpdater
{
    Task UpdateUserAsync(User user);
}

public interface IUserDeleter
{
    Task DeleteUserAsync(int id);
}

// GOOD: Balanced interface segregation
public interface IUserRepository
{
    Task<User> GetByIdAsync(int id);
    Task SaveAsync(User user);
    Task DeleteAsync(int id);
}

public interface IUserQueryService
{
    Task<List<User>> SearchUsersAsync(string criteria);
    Task<UserStatistics> GetUserStatisticsAsync();
}

5. Confusing DIP with DI:

// BAD: Using DI container but not following DIP
public class OrderService
{
    public void ProcessOrder(Order order)
    {
        var repository = ServiceLocator.Get<SqlOrderRepository>(); // Still depends on concrete type
        repository.Save(order);
    }
}

// GOOD: Following DIP with proper abstraction
public class OrderService
{
    private readonly IOrderRepository _repository;

    public OrderService(IOrderRepository repository)
    {
        _repository = repository;
    }

    public void ProcessOrder(Order order)
    {
        _repository.Save(order);
    }
}

Follow-Up Questions You Might Be Asked:

• “How do you balance SOLID with pragmatism?” Tip: Apply SOLID principles where they give real benefits. For simple, stable features, a direct approach might be better. As complexity or change frequency increases, invest more in proper design.

• “What code smells indicate SOLID violations?” Tip: Long methods/classes (SRP), switch statements on types (OCP), unexpected exceptions in subtypes (LSP), unused interface methods (ISP), and direct references to implementations (DIP) are all warning signs.

Common Things People Get Wrong:

• Applying principles mechanically without understanding the problem
• Not considering the team’s skill level and project timeline
• Treating principles as absolute rules rather than guidelines

TL;DR: SOLID principles are guidelines, not laws. Apply them when they solve real problems, not just because they exist.

Too Much of a Good Thing:
Over-abstracting, tiny interfaces, or adding interfaces “just because” , SOLID misapplied can hurt more than help.

11. Explain how SOLID principles help in reducing tight coupling and increasing cohesion

Answer: Coupling and cohesion are fundamental concepts in software design. SOLID principles provide concrete techniques to achieve low coupling and high cohesion.

flowchart TD
    A[Too Many Abstractions] --> B[Interface Explosion]
    A --> C[SRP Confusion]
    A --> D[Over-Engineering]
    A --> E[Inconsistent DIP usage]

    

Tight Coupling Example:

// BAD: High coupling - OrderService knows too much about implementations
public class OrderService
{
    public void ProcessOrder(Order order)
    {
        // Tightly coupled to SQL Server
        using var connection = new SqlConnection("connectionString");
        connection.Open();
        var command = new SqlCommand("INSERT INTO Orders...", connection);
        command.ExecuteNonQuery();

        // Tightly coupled to SMTP
        var smtpClient = new SmtpClient("smtp.gmail.com");
        smtpClient.Send("from@company.com", order.CustomerEmail, "Order Confirmation", "Thank you");

        // Tightly coupled to file system
        File.WriteAllText($"c:\\orders\\{order.Id}.txt", order.ToString());
    }
}

Low Coupling with SOLID:

// GOOD: Low coupling through dependency inversion
public class OrderService
{
    private readonly IOrderRepository _orderRepository;
    private readonly IEmailService _emailService;
    private readonly IOrderAuditService _auditService;

    public OrderService(
        IOrderRepository orderRepository,
        IEmailService emailService,
        IOrderAuditService auditService)
    {
        _orderRepository = orderRepository;
        _emailService = emailService;
        _auditService = auditService;
    }

    public async Task ProcessOrderAsync(Order order)
    {
        await _orderRepository.SaveAsync(order);
        await _emailService.SendOrderConfirmationAsync(order);
        await _auditService.LogOrderProcessedAsync(order);
    }
}

High Cohesion Example:

// GOOD: High cohesion - all methods work together toward a single goal
public class OrderValidator
{
    public ValidationResult Validate(Order order)
    {
        var result = new ValidationResult();
        
        ValidateCustomerInfo(order.Customer, result);
        ValidateOrderItems(order.Items, result);
        ValidatePaymentInfo(order.Payment, result);
        ValidateShippingInfo(order.Shipping, result);
        
        return result;
    }

    private void ValidateCustomerInfo(Customer customer, ValidationResult result)
    {
        if (string.IsNullOrEmpty(customer.Email))
            result.AddError("Customer email is required");
        // Related validation logic
    }

    private void ValidateOrderItems(List<OrderItem> items, ValidationResult result)
    {
        if (!items.Any())
            result.AddError("Order must contain at least one item");
        // Related validation logic
    }

    // Other validation methods...
}

How Each SOLID Principle Helps:

SRP increases cohesion by ensuring each class has a focused purpose:

// High cohesion - all methods relate to user authentication
public class AuthenticationService
{
    public async Task<AuthResult> AuthenticateAsync(string username, string password)
    {
        var user = await ValidateCredentialsAsync(username, password);
        var token = GenerateJwtToken(user);
        await LogAuthenticationAttemptAsync(username, success: true);
        return new AuthResult { Success = true, Token = token };
    }

    private async Task<User> ValidateCredentialsAsync(string username, string password) { /* ... */ }
    private string GenerateJwtToken(User user) { /* ... */ }
    private async Task LogAuthenticationAttemptAsync(string username, bool success) { /* ... */ }
}

DIP reduces coupling by making high-level modules independent of low-level implementations:

// Low coupling - OrderService doesn't know about database technology
public class OrderService
{
    private readonly IOrderRepository _repository; // Abstract dependency

    public OrderService(IOrderRepository repository)
    {
        _repository = repository; // Injected concrete implementation
    }
}

Follow-Up Questions You Might Be Asked:

• “How do you measure coupling in your codebase?” Tip: Look at change patterns - how many files typically change together. Use static analysis tools that measure afferent and efferent coupling. High fan-in or fan-out indicate potential problems.

• “Can you give an example of high coupling that caused problems?” Tip: Describe a real project where a seemingly small change cascaded across multiple components because of tight dependencies. Mention the time impact and bugs it created.

Common Things People Get Wrong:

• Confusing low coupling with no coupling (some coupling is necessary)
• Creating high cohesion by putting everything in one class
• Not recognizing temporal coupling (order of operations matters)

How This Shows Up in Real Life: Use dependency graphs and static analysis tools to identify highly coupled components. Look for classes that change together frequently, they might need better separation of concerns.

TL;DR: SOLID principles systematically reduce coupling (fewer dependencies between components) and increase cohesion (related functionality stays together). This makes code easier to understand, test, and modify.

Loosely Coupled, Tightly Focused:
SOLID helps each part of your system do one thing well , and depend less on everything else. That’s the secret to maintainability.

Don’t create interfaces just to tick a box. Only introduce abstractions when multiple implementations are likely or when testing demands it.

• See LSP: Rectangle vs Square for a full breakdown.
• Learn why DI != DIP.

12. How do you balance SOLID principles with the YAGNI principle in real-world development?

Answer: YAGNI (You Aren’t Gonna Need It) often conflicts with SOLID principles. SOLID encourages abstraction and flexibility, while YAGNI says don’t build what you don’t need yet. The key is knowing when to apply each.

When to Apply YAGNI Over SOLID:

// YAGNI: Simple solution for known, stable requirements
public class ProductService
{
    private readonly ApplicationDbContext _context;

    public ProductService(ApplicationDbContext context)
    {
        _context = context;
    }

    public async Task<Product> GetProductAsync(int id)
    {
        return await _context.Products.FindAsync(id);
    }

    public async Task SaveProductAsync(Product product)
    {
        _context.Products.Add(product);
        await _context.SaveChangesAsync();
    }
}

// Don't create IProductRepository unless you actually need multiple implementations

When to Apply SOLID Despite YAGNI:

// SOLID: Abstract when you know variation is likely
public interface IPaymentProcessor
{
    Task<PaymentResult> ProcessPaymentAsync(decimal amount, PaymentDetails details);
}

public class StripePaymentProcessor : IPaymentProcessor
{
    public async Task<PaymentResult> ProcessPaymentAsync(decimal amount, PaymentDetails details)
    {
        // Stripe implementation
        return new PaymentResult { Success = true };
    }
}

// Even if you only support Stripe today, payment processing is likely to change

Practical Decision Framework:

 flowchart LR
    Start("Do you expect frequent changes?")
    Start -->|Yes| Solid["Apply SOLID"]
    Start -->|No| CRUDCheck{"Is it just CRUD?"}
    CRUDCheck -->|Yes| Yagni["Stick with YAGNI"]
    CRUDCheck -->|No| ComplexCheck{"Complex rules or logic?"}
    ComplexCheck -->|Yes| Solid
    ComplexCheck -->|No| Yagni

    

1. High likelihood of change –> Apply SOLID
2. External dependencies –> Apply SOLID (DIP specifically)
3. Complex business logic –> Apply SOLID (SRP specifically)
4. Simple CRUD operations –> Apply YAGNI
5. Internal utilities –> Apply YAGNI

Evolution Example:

// Version 1: YAGNI approach
public class NotificationService
{
    public async Task SendWelcomeEmailAsync(string email, string userName)
    {
        var smtpClient = new SmtpClient("smtp.company.com");
        await smtpClient.SendAsync(email, "Welcome!", $"Hello {userName}!");
    }
}

// Version 2: Requirements changed, refactor to SOLID
public interface INotificationService
{
    Task SendNotificationAsync(string recipient, string subject, string message, NotificationType type);
}

public class NotificationService : INotificationService
{
    private readonly IEmailService _emailService;
    private readonly ISmsService _smsService;
    private readonly IPushNotificationService _pushService;

    public NotificationService(
        IEmailService emailService,
        ISmsService smsService,
        IPushNotificationService pushService)
    {
        _emailService = emailService;
        _smsService = smsService;
        _pushService = pushService;
    }

    public async Task SendNotificationAsync(string recipient, string subject, string message, NotificationType type)
    {
        switch (type)
        {
            case NotificationType.Email:
                await _emailService.SendAsync(recipient, subject, message);
                break;
            case NotificationType.Sms:
                await _smsService.SendAsync(recipient, message);
                break;
            case NotificationType.Push:
                await _pushService.SendAsync(recipient, subject, message);
                break;
        }
    }
}

Signs It’s Time to Refactor from YAGNI to SOLID:

• You’re modifying the same class for different reasons (violating SRP)
• You need to mock dependencies for testing
• You’re copying similar code across multiple classes
• Business requirements suggest future variations

Follow-Up Questions You Might Be Asked:

• “How do you decide when to refactor from simple to SOLID?” Tip: Watch for pain points like difficult testing, duplicate code, or frequent changes to the same file for different reasons. When a simple class gets too many responsibilities, that’s your cue to refactor.

• “What’s an example where YAGNI saved you from over-engineering?” Tip: Share a story where you started simple and it was sufficient. For example, using direct database access for a simple admin CRUD app that never needed alternative implementations or complex testing.

Common Things People Get Wrong:

• Using YAGNI as an excuse for poor design
• Over-engineering based on hypothetical future requirements
• Not refactoring when YAGNI assumptions prove wrong

TL;DR: Start with YAGNI for simple, stable requirements. Refactor to SOLID when you hit real complexity or variation. The key is recognizing when your YAGNI assumptions are wrong.

Future-Proof vs Over-Engineering:
SOLID builds flexibility, YAGNI keeps things lean. Use SOLID where you expect change. Stick to YAGNI where things are stable.

How to Prepare for SOLID Questions in Interviews

Study Resources:

• Clean Code by Robert C. Martin - The definitive guide to SOLID principles
• Clean Architecture by Robert C. Martin - Shows SOLID at system scale
• Refactoring by Martin Fowler - Practical techniques for improving design
• Design Patterns by Gang of Four - Shows how patterns implement SOLID

Hands-On Practice:

• Take existing code and identify SOLID violations
• Practice explaining principles without using jargon
• Build small projects that demonstrate each principle
• Refactor legacy code to follow SOLID principles

Interview Preparation Tips:

• Have concrete examples ready from your own experience
• Practice drawing simple UML diagrams
• Be prepared to discuss trade-offs and when to break principles
• Know the difference between principles and patterns

Conclusion

SOLID principles aren’t just interview topics, they’re practical tools for writing maintainable, testable code. The key is understanding when to apply them and when simpler solutions are better.

Don’t memorize definitions. Instead, focus on the problems these principles solve: tight coupling, low cohesion, and code that’s hard to change. When you encounter these problems in real projects, you’ll naturally reach for SOLID solutions.

Remember that principles are guidelines, not laws. A well-designed system that solves real problems is better than a perfectly SOLID system that over-engineers simple solutions.

Practice applying these principles in your daily work. Start with small refactoring exercises, then gradually apply them to larger system design decisions. The more you use SOLID principles in real scenarios, the more natural they’ll become in interviews and in your career as a developer.

The goal isn’t to impress interviewers with fancy terminology, it’s to demonstrate that you can build systems that your future teammates will thank you for.

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